The use of polyacrylonitrile (PAN) to make carbon fibers is well known in the art. PAN filaments, generally in the form of multifilament tows, are treated to stabilize the filaments prior to ultimately carbonizing the tows at temperatures which usually range from about 650.degree. to about 1650.degree. C. The stabilization treatment involves heating multifilament tows in an oxygen-containing atmosphere at a temperature of about 180.degree. to about 400.degree. C. Without such a stabilization treatment the fibers which ultimately result exhibit significantly inferior physical properties, particularly tensile strength.
Prior to undergoing stabilization and carbonization, tows are generally mechanically treated to maintain the filaments comprising them as integral bundles during stabilization and carbonization. The stabilization process in particular generally involves numerous mechanical manipulations whereby tows are in physical contact with guides, rollers, and the like such that significant opportunities for damage to the tows exist. PAN tows heated in the temperature range of stabilization soften and rub or drag on the stabilization processing equipment and are damaged through the generation of breaks in individual filaments which weakens the tow or through the development of caternaries (also known as loop-outs) which cause unacceptable visual quality in the carbon fibers which ultimately result. Consequently, a standard practice is to air-interlace the tows to bundle them. Air-interlacing effects a "controlled entanglement" which is intended to prevent the filaments from splaying out during the stabilization, carbonization, or other procedures involving physical manipulation. Tows which are not integrally bundled are frequently and, sometimes, excessively damaged leading, as mentioned, to frequent wrap-ups, loop-outs, and unacceptable visual quality due to "fuzziness". However, air-interlacing itself represents a tradeoff since, by entangling the filaments within a tow, some fuzziness and other forms of damage result, even though it is their occurrence which is sought to be minimized.
An alternative to interlacing is twisting the tow to maintain it as an integral, linear bundle. Twisting, however, can also inherently damage the bundle and cause fuzziness through distortion of the filaments in the act of twisting off axis. Tows can be also damaged by breaking individual filaments therein. The twisting further imparts a permanent (twisted) set to the tows, and thus requires the tows to be untwisted. The machinery required to accomplish this is generally fairly sophisticated and very expensive. Moreover, retrofits to existing continuous carbon fiber production operations are not easily accomplished to incorporate twisting, in addition to the significant expense involved.
A method of maintaining PAN tows as integral bundles without the need to entangle the filaments therein or to twist the tows would thus be a useful addition to the art of forming carbon fibers.